Locomotor activity and non-photic influences on circadian clocks

Pubertal development of sex differences in circadian function: an animal model

The development of adult circadian function, particularly sexual dimorphism of function, has been well studied only in rapidly developed rodents. In such species development is complete by weaning. Data from adolescent humans suggest that significant development occurs during the pubertal period. We hypothesized that a more slowly developing rodent might better mimic the changes in circadian function around puberty in humans and allow us to determine the underlying neural changes. Entrained and free-running circadian rhythms were analyzed and correlated with pubertal development in male and female Octodon degus (degu) that remained gonadally intact or were gonadectomized at weaning. Brains were collected during development to measure androgen and estrogen receptors in the suprachiasmatic nuclei (SCN) Adult circadian period does not develop until 10-12 months of age in degus, long after the onset of gonadal maturation (3-5 months). The timing of circadian period maturation correlates with the appearance of steroid receptors in the SCN. Changes in free-running rhythms only occurred in gonadally intact degus. Adult phase angles of activity onset develop between 2 and 3 months of age (comparing results of two experiments), soon after the onset of pubertal changes.

CONCLUSION

The development of sexually dimorphic adult circadian period occurs after gonadal puberty is complete and requires the presence of gonadal steroids. The delay in development until after gonadal puberty is likely due to the delayed appearance of steroid receptors in the SCN. Phase is not sexually dimorphic and changes in the absence of steroid hormones.

Social influences on mammalian circadian rhythms: animal and human studies

While light is considered the dominant stimulus for entraining (synchronizing) mammalian circadian rhythms to local environmental time, social stimuli are also widely cited as 'zeitgebers' (time-cues). This review critically assesses the evidence for social influences on mammalian circadian rhythms, and possible mechanisms of action. Social stimuli may affect circadian behavioural programmes by regulating the phase and period of circadian clocks (i.e. a zeitgeber action, either direct or by conditioning to photic zeitgebers), by influencing daily patterns of light exposure or modulating light input to the clock, or by associative learning processes that utilize circadian time as a discriminative or conditioned stimulus. There is good evidence that social stimuli can act as zeitgebers. In several species maternal signals are the primary zeitgeber in utero and prior to weaning. Adults of some species can also be phase shifted or entrained by single or periodic social interactions, but these effects are often weak, and appear to be mediated by social stimulation of arousal. There is no strong evidence yet for sensory-specific nonphotic inputs to the clock. The circadian phase-dependence of clock resetting to social stimuli or arousal (the 'nonphotic' phase response curve, PRC), where known, is distinct from that to light and similar in diurnal and nocturnal animals. There is some evidence that induction of arousal can modulate light input to the clock, but no studies yet of whether social stimuli can shift the clock by conditioning to photic cues, or be incorporated into the circadian programme by associative learning. In humans, social zeitgebers appear weak by comparison with light. In temporal isolation or under weak light-dark cycles, humans may ignore social cues and free-run independently, although cases of mutual synchrony among two or more group-housed individuals have been reported. Social cues may affect circadian timing by controlling sleep-wake states, but the phase of entrainment observed to fixed sleep-wake schedules in dim light is consistent with photic mediation (scheduled variations in behavioural state necessarily create daily light-dark cycles unless subjects are housed in constant dark or have no eyes). By contrast, discrete exercise sessions can induce phase shifts consistent with the nonphotic PRC observed in animal studies. The best evidence for social entrainment in humans is from a few totally blind subjects who synchronize to the 24 h day, or to near-24 h sleep-wake schedules under laboratory conditions. However, the critical entraining stimuli have not yet been identified, and there are no reported cases yet of social entrainment in bilaterally enucleated blind subjects. The role of social zeitgebers in mammalian behavioural ecology, their mechanisms of action, and their utility for manipulating circadian rhythms in humans, remains to be more fully elaborated.

The role of Period1 in non-photic resetting of the hamster circadian pacemaker in the suprachiasmatic nucleus

Non-photic stimuli, such as diurnal wheel running in rodents, phase shift the circadian clock and suppress the expression of Per1 in the suprachiasmatic nucleus (SCN). The goal of the present study was to directly decrease Per1 expression using antisense (AS) oligodeoxynucleotides to determine if such suppression produced non-photic phase shifts. Injections of Per1-AS suppressed expression of Per1 within the SCN and produced phase shifts similar to those resulting from other non-photic manipulation, with large phase advances to injections during the subjective day. These results indicate that the decrease in expression of Per1 is a cause rather than a consequence of non-photic phase shifts.

Intergeniculate leaflet and ventral lateral geniculate nucleus afferent connections: An anatomical substrate for functional input from the vestibulo-visuomotor system

The intergeniculate leaflet (IGL) has widespread projections to the basal forebrain and visual midbrain, including the suprachiasmatic nucleus (SCN). Here we describe IGL-afferent connections with cells in the ventral midbrain and hindbrain. Cholera toxin B subunit (CTB) injected into the IGL retrogradely labels neurons in a set of brain nuclei most of which are known to influence visuomotor function. These include the retinorecipient medial, lateral and dorsal terminal nuclei, the nucleus of Darkschewitsch, the oculomotor central gray, the cuneiform, and the lateral dorsal, pedunculopontine, and subpeduncular pontine tegmental nuclei. Intraocular CTB labeled a retinal terminal field in the medial terminal nucleus that extends dorsally into the pararubral nucleus, a location also containing cells projecting to the IGL. Distinct clusters of IGL-afferent neurons are also located in the medial vestibular nucleus. Vestibular projections to the IGL were confirmed by using anterograde tracer injection into the medial vestibular nucleus. Other IGL-afferent neurons are evident in Barrington's nucleus, the dorsal raphe, locus coeruleus, and retrorubral nucleus. Injection of a retrograde, trans-synaptic, viral tracer into the SCN demonstrated transport to cells as far caudal as the vestibular system and, when combined with IGL injection of CTB, confirmed that some in the medial vestibular nucleus polysynaptically project to the SCN and monosynaptically to the IGL, as do cells in other brain regions. The results suggest that the IGL may be part of the circuitry governing visuomotor activity and further indicate that circadian rhythmicity might be influenced by head motion or visual stimuli that affect the vestibular system.

Entrainment of the master circadian clock by scheduled feeding

The master circadian clock, located in the mammalian suprachiasmatic nuclei (SCN), generates and coordinates circadian rhythmicity, i.e., internal organization of physiological and behavioral rhythms that cycle with a near 24-h period. Light is the most powerful synchronizer of the SCN. Although other nonphotic cues also have the potential to influence the circadian clock, their effects can be masked by photic cues. The purpose of this study was to investigate the ability of scheduled feeding to entrain the SCN in the absence of photic cues in four lines of house mouse (Mus domesticus). Mice were initially housed in 12:12-h light/dark cycle with ad libitum access to food for 6 h during the light period followed by 4-6 mo of constant dark under the same feeding schedule. Wheel running behavior suggested and circadian PER2 protein expression profiles in the SCN confirmed entrainment of the master circadian clock to the onset of food availability in 100% (49/49) of the line 2 mice in contrast to only 4% (1/24) in line 3 mice. Mice from line 1 and line 4 showed intermediate levels of entrainment, 57% (8/14) and 39% (7/18), respectively. The predictability of entrainment vs. nonentrainment in line 2 and line 3 and the novel entrainment process provide a powerful tool with which to further elucidate mechanisms involved in entrainment of the SCN by scheduled feeding.

A circadian rhythm in the expression of PERIOD2 protein reveals a novel SCN-controlled oscillator in the oval nucleus of the bed nucleus of the stria terminalis

Circadian rhythms in mammals are regulated not only globally by the master clock in the suprachiasmatic nucleus (SCN), but also locally by widely distributed populations of clock cells in the brain and periphery that control tissue-specific rhythmic outputs. Here we show that the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV) exhibits a robust circadian rhythm in expression of the Period2 (PER2) clock protein. PER2 expression is rhythmic in the BNST-OV in rats housed under a light/dark cycle or in constant darkness, in blind rats, and in mice, and is in perfect synchrony with the PER2 rhythm of the SCN. Constant light or bilateral SCN lesions abolish the rhythm of PER2 in the BNST-OV. Large abrupt shifts in the light schedule transiently uncouple the BNST-OV rhythm from that of the SCN. Re-entrainment of the PER2 rhythm is faster in the SCN than in the BNST-OV, and it is faster after a delay than an advance shift. Bilateral adrenalectomy blunts the PER2 rhythm in the BNST-OV. Thus, the BNST-OV contains circadian clock cells that normally oscillate in synchrony with the SCN, but these cells appear to require both input from the SCN and circulating glucocorticoids to maintain their circadian oscillation. Taken together with what is known about the functional organization of the connections of the BNST-OV with systems of the brain involved in stress and motivational processes, these findings place BNST-OV oscillators in a position to influence specific physiological and behavioral rhythms downstream from the SCN clock.

Sharing time on the fly

The investigation of circadian clock function in Drosophila has progressed from the identification of clock genes to the analysis of timing mechanisms in the cells and tissues where these genes are expressed. As the biological context for investigating circadian clock systems is expanded, new features of molecular timing mechanisms are becoming apparent. Examples come first from studies on peripheral clocks, which perform local, tissue-specific functions as well as global functions that relate to the control of individual behavior, and second from the evaluation of social influences on circadian rhythms. The identification of inter-organismal components of the circadian system in Drosophila suggests new perspectives as the progression continues from the systems level to the social level and onwards to the level of ecosystems.

Octodon degus: a diurnal, social, and long-lived rodent

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.

Reward and Aversive Stimuli Produce Similar Nonphotic Phase Shifts

Circadian rhythms in rodents respond to arousing, nonphotic stimuli that contribute to daily patterns of entrainment. To examine whether the motivational significance of a stimulus is important for eliciting nonphotic circadian phase shirts in Syrian hamsters (Mesocricetus auratus), the authors compared responses to a highly rewarding stimulus (lateral hypothalamic brain stimulation reward [BSR]) and a highly aversive stimulus (footshock). Animals were housed on a 14:10-hr light-dark cycle until test day, when they were given a 1-hr BSR session (trained animals) or a 1-mA electric footshock at 1 of 8 circadian times, and were maintained in constant dark thereafter. Both BSR pulses and footshock produced nonphotic phase response curves. These results support the hypothesis that arousal resulting from the motivational significance of a stimulus is a major factor in nonphotic phase shifts.

Dark pulse resetting of the suprachiasmatic clock in Syrian hamsters: behavioral phase-shifts and clock gene expression

In mammals, the circadian clock in the suprachiasmatic nuclei (SCN) is mainly synchronized to photic cues provided by the daily light/dark cycle. Phase-shifts produced by light exposure during the night are correlated with rapid induction of two clock genes, Per1 and Per2, in the SCN. Nonphotic stimuli such as behavioral and pharmacological cues, when presented during the subjective day, induce behavioral phase-advances and a down-regulation of Per1 and Per2 expression in the SCN. When applied during the subjective day, dark pulses in continuous light also produce phase-advances. These phase-shifting effects have been interpreted as reflecting either a photic image mirror, nonphotic cues, or a combination of both. Here we evaluated in Syrian hamsters housed in constant light how dark pulses applied in late subjective day affect levels of Per1, Per2 and Cry1 mRNA. Four-hour dark pulses with no access to a wheel produced 1.2+/-0.4 h phase-advances of locomotor activity rhythm while control manipulation induced non-significant shifts (0.1+/-0.2 h). Dark pulses transiently down-regulated Per1 and Per2 mRNA levels in the SCN by 40 and 20% respectively, while the levels of Cry1 mRNA remained unaffected. In behaviorally split hamsters in which Per oscillations were asymmetric between the left and right sides of the SCN, dark pulses reduced Per expression in the half-SCN with high Per. This study shows that exposure during the late subjective day to dark pulses independent of wheel-running have nonphotic-like effects on the SCN clock at both behavioral and molecular levels.

Crossed and uncrossed retinal projections to the hamster circadian system

The hamster suprachiasmatic nucleus (SCN), site of the circadian clock, has been thought to be equally and completely innervated by each retina. This issue was studied in animals that had received an injection of the tracer cholera toxin subunit B (CTb) conjugated to Alexa 488 into the vitreous of one eye, with CTb-Alexa 594 injected into the other. Retinal projections to the SCN and other nuclei of the circadian system were simultaneously evaluated by using confocal laser microscopy. Each retina provides completely overlapping terminal fields throughout each SCN. Although SCN innervation by the contralateral retina is slightly denser than that from the ipsilateral retina, there are distinct SCN regions where input from one side is predominant, but not exclusive. A dense terminal field from the contralateral retina encompasses, and extends dorsally beyond, the central SCN subnucleus identified by calbindin-immunoreactive neurons. Surrounding the dense terminal field, innervation is largely derived from the ipsilateral retina. The densest terminal field in the intergeniculate leaflet is from the contralateral retina, which completely overlaps the ipsilateral projection. Most nuclei of the pretectum receive innervation largely, but not solely, from the contralateral retina, although the olivary pretectal nucleus has very dense patches of innervation derived exclusively from one retina or the other. Retina-dependent variation in terminal field density within the three closely examined nuclei may indicate areas of specialized function not previously appreciated. This issue is discussed in the context of the melanopsin-containing retinal ganglion cell projections to several nuclei in the circadian visual system.

Photoperiod differentially modulates photic and nonphotic phase response curves of hamsters

Circadian pacemakers respond to light pulses with phase adjustments that allow for daily synchronization to 24-h light-dark cycles. In Syrian hamsters, Mesocricetus auratus, light-induced phase shifts are larger after entrainment to short daylengths (e.g., 10 h light:14 h dark) vs. long daylengths (e.g., 14 h light:10 h dark). The present study assessed whether photoperiodic modulation of phase resetting magnitude extends to nonphotic perturbations of the circadian rhythm and, if so, whether the relationship parallels that of photic responses. Male Syrian hamsters, entrained for 31 days to either short or long daylengths, were transferred to novel wheel running cages for 2 h at times spanning the entire circadian cycle. Phase shifts induced by this stimulus varied with the circadian time of exposure, but the amplitude of the resulting phase response curve was not markedly influenced by photoperiod. Previously reported photoperiodic effects on photic phase resetting were verified under the current paradigm using 15-min light pulses. Photoperiodic modulation of phase resetting magnitude is input specific and may reflect alterations in the transmission of photic stimuli.

Circadian clock resetting by arousal in Syrian hamsters: the role of stress and activity

Circadian rhythms in the Syrian hamster can be markedly phase shifted by 3 h of wheel running or arousal stimulation during their usual daily rest period ("subjective day"). Continuous wheel running is predictive but not necessary for phase shifts of this "nonphotic" type; hamsters aroused by gentle handling without running can also show maximal shifts. By contrast, physical restraint, a standard stress procedure and thus presumably arousing, is ineffective. To resolve this apparent paradox, phase-shifting effects of 3-h sessions of restraint or other stress procedures were assessed. In a preliminary study, phase shifts to arousal by gentle handling were significantly potentiated by the cortisol synthesis inhibitor metyrapone, suggesting that stress-related cortisol release may inhibit phase shifts to arousal. Next, it was confirmed that restraint in the subjective day does not induce phase shifts, but behavioral observations revealed that it also does not sustain arousal. Restraint combined with noxious compressed air blasts did sustain arousal and induced a significant cortisol response compared with arousal by gentle handling but did not induce shifts. Restraint combined with continuous horizontal rotation was also ineffective, as was EEG-validated arousal via confinement to a pedestal over water. However, 3 h of resident-intruder interactions (an intense psychosocial stress) or exposure to an open field (a mild stress) did induce large shifts that were positively correlated with indexes of forward locomotion. The results indicate that large phase shifts associated with arousal in the usual sleep period are neither induced nor prevented by stress per se, but are dependent on the expression of at least low levels of locomotor activity. Sustained arousal alone is not sufficient.

A broad role for melanopsin in nonvisual photoreception

The rod and cone photoreceptors that mediate visual phototransduction in mammals are not required for light-induced circadian entrainment, negative masking of locomotor activity, suppression of pineal melatonin, or the pupillary light reflex. The photopigment melanopsin has recently been identified in intrinsically photosensitive retinal ganglion cells (RGCs) that project to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus, suggesting that melanopsin might influence a variety of irradiance-driven responses. We have found novel projections from RGCs that express melanopsin mRNA to the ventral subparaventricular zone (vSPZ), a region involved in circadian regulation and negative masking, and the sleep-active ventrolateral preoptic nucleus (VLPO) and determined the subsets of melanopsin-expressing RGCs that project to the SCN, the pretectal area (PTA), and the IGL division of the lateral geniculate nucleus (LGN). Melanopsin was expressed in the majority of RGCs that project to the SCN, vSPZ, and VLPO and in a subpopulation of RGCs that innervate the PTA and the IGL but not in RGCs projecting to the dorsal LGN or superior colliculus. Two-thirds of RGCs containing melanopsin transcript projected to each of the SCN and contralateral PTA, and one-fifth projected to the ipsilateral IGL. Double-retrograde tracing from the SCN and PTA demonstrated a subpopulation of RGCs projecting to both sites, most of which contained melanopsin mRNA. Our results suggest that melanopsin expression defines a subset of RGCs that play a broad role in the regulation of nonvisual photoreception, providing collateralized projections that contribute to circadian entrainment, negative masking, the regulation of sleep-wake states, and the pupillary light reflex.

Behavioral arousal blocks light-induced phase advances in locomotor rhythmicity but not light-induced Per1 and Fos expression in the hamster suprachiasmatic nucleus

Both photic and nonphotic stimuli entrain circadian rhythms. Although the adaptive significance of nonphotic clock resetting is unknown, one possibility is that nonphotic cues modulate circadian responses to light. Results of studies on the interaction between photic and nonphotic stimuli support this idea. During the day, light blocks the effects of nonphotic stimuli on the phase of locomotor rhythms and on expression of clock genes in suprachiasmatic nucleus (SCN) neurons. At night, novelty-induced activity prior to and during exposure to light attenuates the phase-shifting response to that light, but the effects of this manipulation on clock gene expression are unknown. The present experiments explore the interaction between behavioral state and response to light at the molecular level. We show that confining hamsters to novel wheels immediately after a light pulse during the late subjective night attenuates light-induced phase advances of wheel-running rhythms and the transient effects on circadian period. In contrast to the striking effect of novelty-induced activity on behavioral responses to light, Fos protein and Per1 mRNA were robustly expressed in the SCN of all light-pulsed animals, regardless of behavioral treatment. Our results are inconsistent with the idea that light and nonphotic stimuli block each other's effects on phase shifts by inducing or attenuating transcription of Per1. Photic regulation of clock genes and spontaneous rhythmic expression of clock genes are probably mediated by different mechanisms.

Nonphotic phase shifting in female Syrian hamsters: interactions with the estrous cycle

Nonphotic phase shifting of circadian rhythms was examined in female Syrian hamsters. Animals were stimulated at zeitgeber time 4.5 by either placing them in a novel running wheel or by transferring them to a clean home cage. Placement in a clean home cage was more effective than novel wheel treatment in stimulating large (> 1.5 h) phase shifts. Peak phase shifts (ca. 3.5 h) and the percentage of females showing large phase shifts were comparable to those found in male hamsters stimulated with novel wheels. The amount of activity induced by nonphotic stimulation and the amount of phase shifting varied slightly with respect to the 4-day estrous cycle. Animals tended to run less and shift less on the day of estrus. Nonphotic stimulation on proestrus often resulted in a 1-day delay of the estrous cycle reflected in animals' postovulatory vaginal discharge and the expression of sexual receptivity (lordosis). This delay of the estrous cycle was associated with large phase advances and high activity. These results extend the generality of nonphotic phase shifting to females for the first time and raise the possibility that resetting of circadian rhythms can induce changes in the estrous cycle.

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Circadian phase-shifting effects of nocturnal exercise in older compared with young adults

Exercise can phase shift the circadian rhythms of young adults if performed at the right time of day. Similar research has not been done in older adults. This study examined the circadian phase-delaying effects of a single 3-h bout of low-intensity nocturnal exercise in older (n = 8; 55-73 yr old) vs. young (n = 8; 20-32 yr old) adults. The exercise occurred at the beginning of each subject's habitual sleep time, and subjects sat in a chair in dim light during the corresponding time in the control condition. The dim-light melatonin onset (DLMO) was used as the circadian phase marker. The DLMO phase delayed more after the exercise than after the control condition. On average, the difference in phase shift between the exercise and control conditions was similar for older and young subjects, demonstrating that the phase-shifting effects of exercise on the circadian system are preserved in older adults. Therefore, exercise may potentially be a useful treatment to help adjust circadian rhythms in older and young adults.

Circadian activity rhythms in high-alcohol-preferring and low-alcohol-preferring mice

The circadian periods of high-alcohol-preferring (HAP) and low-alcohol-preferring (LAP) selected lines of mice were compared. The mice were ethanol-naive. Circadian periods were calculated from records of running-wheel activity in constant dark. The number of daily wheel revolutions and body weights of the two lines of mice were also compared. The HAP line had a shorter period of wheel running than that of the LAP lines. The HAP mice also had a tendency to run more on wheels than did LAP mice. These findings support the suggestion that genes affecting ethanol consumption in mice have pleiotropic effects on circadian period.

Periodic absence of nursing mothers phase-shifts circadian rhythms of clock genes in the suprachiasmatic nucleus of rat pups

Effects of absence of nursing mothers on the circadian pacemaker of their offspring were examined by measuring clock genes, the rat Per1 (rPer1) and rPer2 expression rhythms in the pup suprachiasmatic nuclei (SCN). Neonate rats born to mothers kept under a 12-h light : 12-h dark cycle (LD) were blinded immediately after birth and exposed to periodic maternal deprivation where pups were deprived of their mothers during the light phase of 12-h for the first week of life. At postnatal day 6, the periodic maternal deprivation completely phase-reversed the circadian rhythms in expression of the clock genes in the pup SCN and in spontaneous locomotor activity after the pups were weaned at postnatal day 21. The periodic maternal absence also altered the patterns of stress-related gene expressions such as corticotropine-releasing hormone, arginine vasopressin, and glucocorticoid receptor in particular brain areas of the mother-deprived pups at P6. These findings indicate that periodic absence of the nursing mother in the first week of life produces a resetting effect on the neonatal circadian clock and induces stress responses in the hypothalamus-pituitary-adrenal axis.

Dorsal raphe nucleus modulates neuronal activity in rat intergeniculate leaflet

Serotonergic input from midbrain raphe nuclei is believed to have a significant effect on mammalian circadian timing system. The suprachiasmatic nucleus (SCN) receives its serotonergic input from the median raphe nucleus, while the intergeniculate leaflet (IGL) receives serotonergic innervation from the dorsal raphe nucleus (DRN). The present paper was aimed at determining whether projection from the DRN affected rhythmic neuronal oscillations in the IGL of rats. We investigated the impact of electrolytic lesions and electric stimulation of the DRN on spontaneous isoperiodic (i.e. burst firing with a constant interburst interval) neuronal activity recorded in the IGL. In all our experiments a complete lesion of the DRN always caused a significant increase (ca. 100%) of spontaneous activity of IGL neurons, their oscillatory character having been maintained, though. On the other hand, electric stimulation of the DRN produced a transient decrease in firing rate oscillations of the IGL neurons. The obtained results indicate that the neuronal projection from the DRN has a substantial modulating effect on IGL activity-an important element of the mechanism of the circadian time-keeping system that mediates the transfer of non-photic information to the SCN by modulating its activity. The observed increase of isoperiodic activity in the IGL after DRN lesion and a transient decrease in this activity after electric stimulation indicate an inhibitory character of this effect. The present findings corroborate the hypothesis that the DRN is a one of the major and extremely important source of the modulatory inputs to the mammalian circadian time-keeping system.

Afferent projections to the hamster intergeniculate leaflet demonstrated by retrograde and anterograde tracing

The intergeniculate leaflet (IGL) is considered involved in nonphotic shifting of the circadian clock through a direct connection, the geniculo-hypothalamic tract. The brain areas mediating nonphotic arousal to the hamster IGL have not been thoroughly investigated by both retrograde and anterograde tracing. We, therefore, reinvestigated the IGL afferent connections with the retrograde tracer Cholera toxin B and subsequently verified the results with the anterograde tracer Phaseolus vulgaris-leucoagglutinin. We also defined a subset of neurons projecting to the IGL that were activated by arousal using c-Fos immunocytochemistry. Apart from a dense afferent projection from the retina- and the contralateral leaflet, there were ipsilateral projections from other structures: layer V and VI of the prefrontal cortex, the zona incerta, the magnocellular part of the subparafascicular nucleus, the dorsal raphe nucleus, the locus coeruleus, and the cuneiform nucleus. Dense bilateral projections to the leaflet from the pretectal nuclei were found. Hypothalamic afferents were observed dorsal to the suprachiasmatic nuclei, in the retrochiasmatic area (RCh) and in the ventromedial hypothalamic nuclei. All of these projections were confirmed by anterograde tracing. Furthermore, arousal (wheel-running) induced c-Fos in neurons projecting to the IGL (prefrontal cortex, RCh, pretectum). Taken together, the data strengthen the view that the IGL integrates photic and nonphotic information.

The circadian system of Trypanosoma cruzi-infected mice

The effects of Chagas disease on the mammalian circadian system were studied in Trypanosoma cruzi-infected C57-B16J mice. Animals were inoculated with CAI or RA strains of T. cruzi or vehicle, parasitism confirmed by blood specimen visualization and locomotor activity rhythms analyzed by wheel-running recording. RA-strain infected mice exhibited significantly decreased amplitude of circadian rhythms, both under light-dark and constant dark conditions, probably due to motor deficiencies. CAI-treated animals showed normal locomotor activity rhythms. However, in these mice, reentrainment to a 6h phase shift of the LD cycle took significantly longer than controls, and application of 15min light pulses in DD produced smaller phase delays of the rhythms. All groups exhibited light-induced Fos expression in the suprachiasmatic nuclei. We conclude that the main effect of T. cruzi infection on the circadian system is an impairment of the motor output from the clock toward controlled rhythms, together with an effect on circadian visual sensitivity.

Effect of cage enrichment on the daily use of running wheels by Syrian hamsters

Institutional animal care committees may one day require for the welfare of captive hamsters more floor space and the introduction of tunnels and toys. As hamsters are popular animal subjects in chronobiological research, and as clock phase is usually measured through running wheel activity, it is important to determine what effect cage enrichment might have on daily wheel use. Here the daily number of wheel revolutions, the daily duration of the running activity phase, the phase relationship between lights-off and onset of running activity, and the free-running period of circadian activity rhythms were measured in Syrian hamsters, Mesocricetus auratus, housed in single cages or in multiple cages linked by tunnels and supplied with commercial wooden toys. Free-running periodicity was not affected by cage enrichment. In multiple-cage systems, there were fewer daily revolutions, shorter wheel-running activity phases, and delayed running activity onsets. These effects, however, were small as compared to interindividual and week-to-week variation. They were statistically significant only under a light:dark cycle, not in constant darkness, and only when interindividual variation was eliminated through a paired design or when the number of cages was increased to five (the maximum tested). Daily wheel use is thus affected by cage enrichment, but only slightly.

Do circadian rhythms in respiratory control contribute to sleep-related breathing disorders?

Sleep-related respiratory dysfunction compromises the health and quality of life of millions of people worldwide, underscoring the need for a full understanding of the mechanisms by which the respiratory control system is altered at night. This paper suggests the hypothesis that the circadian timing system may play a role in the pathogenesis of some types of sleep-related breathing disorders. Recent studies have provided evidence that the circadian timing system has an influence on respiration and respiratory control, even in the absence of sleep. These new data are reviewed and potential mechanisms underlying the circadian modulation of breathing are outlined, identifying important gaps in our knowledge. It is proposed that circadian rhythms in respiratory control may increase the propensity for nocturnal respiratory instability and recurrent apnea. Importantly, circadian and sleep mechanisms appear to have additive effects on breathing, suggesting that the circadian timing system can potentially amplify or suppress sleep-related breathing abnormalities, depending upon the characteristics of the circadian output and the time of day at which sleep occurs.

Attenuation of phase shifts to light by activity or neuropeptide Y: a time course study

Circadian rhythms in mammals can be synchronised to photic and non-photic stimuli. Interactions between photic and behavioural stimuli were investigated during the late subjective night, 6 h after activity onset in Syrian hamsters (CT18). Light pulses of 130 lx for 15 min at this time resulted in phase advance shifts. Novel wheel exposure, for a period of 3 h, following photic stimulation was able to attenuate the phase advancing effects of light. A time delay of up to 60 min between photic and behavioural stimuli also resulted in significant attenuation of light-induced phase shifts (P<0.05). A 90-min interval between stimuli resulted in no significant attenuation. Novel wheel exposure mediates its effects via the intergeniculate leaflet, which conveys information to the SCN and utilises neuropeptide Y (NPY) as its primary neurotransmitter. Phase shifts to light pulses given at CT18 were attenuated by NPY administration. Neuropeptide Y injections up to 60 min post-light exposure significantly attenuated phase shifts by 50% on average. However a 90-min interval between light and NPY microinjection did not significantly affect light-induced phase shifts. These results confirm previous work indicating that novel wheel exposure and NPY administration can modulate light-induced phase shifts during the late night. Further, they show for the first time that the time course for this interaction is similar between wheel running and NPY. Most significantly, our work indicates that the time course in vivo in the late night is similar to that shown previously in vitro during the early night.

Resetting the circadian clock by social experience in Drosophila melanogaster

Circadian clocks are influenced by social interactions in a variety of species, but little is known about the sensory mechanisms underlying these effects. We investigated whether social cues could reset circadian rhythms in Drosophila melanogaster by addressing two questions: Is there a social influence on circadian timing? If so, then how is that influence communicated? The experiments show that in a social context Drosophila transmit and receive cues that influence circadian time and that these cues are likely olfactory.

Placing ocular mutants into a functional context: a chronobiological approach

The behavior of mammals is characterized by a 24-h cycle of rest and activity which is a fundamental adaption to the solar cycle of light and darkness. The pacemaker of this circadian clock is localized in the ventral part of the hypothalamus, the so-called suprachiasmatic nuclei (SCN), and is entrained by light signals mediated by the eye. The eye is directly connected via the retinohypothalamic tract (RHT) to the SCN. Light that reaches the retina elicits glutamate release at the synaptic terminals of the RHT and influences the neurons in the SCN in a manner that alters the behavioral state of the animal. A light pulse that reaches the retina at the beginning of the night elicits a delay of the clock phase, whereas a light pulse that reaches the retina at the end of the dark period leads to an advance of the clock phase. This advance or delay can be quantified by measuring the change in onset of wheel-running activity. Such measurements have, and continue to provide, a remarkably powerful assay of how light is detected and transduced to regulate circadian rhythms. The methods used for such measurements in mice are described in the following article.

MDMA alters the response of the circadian clock to a photic and non-photic stimulus

3,4-Methylenedioxymethamphetamine (MDMA or 'Ecstasy') is a widely used recreational drug that damages serotonin 5-HT neurons in animals and possibly humans. Published literature has shown that the serotonergic system is involved in photic and non-photic phase shifting of the circadian clock, which is located in the suprachiasmatic nuclei. Despite the dense innervation of the circadian system by 5-HT and the known selective neurotoxicity of MDMA, little is known about the effects of MDMA on the circadian oscillator. This study investigated whether repeated exposure to the serotonin neurotoxin MDMA alters the behavioural response of the Syrian hamster to phase shift to the serotonin 5-HT1A/7 receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). This agonist was administered under an Aschoff Type I (CT8) and Aschoff Type II (ZT8) paradigm (5 mg/kg) and was given before and after treatment with MDMA (10, 15 and 20 mg/kg administered on successive days). Pre-treatment with MDMA significantly attenuated phase shifts to 8-OH-DPAT. We also tested the ability of the clock to phase shift to a photic stimulus after treatment with MDMA. A 15-min light pulse (mean lux 125 at CT14 or ZT14) was administered before and after treatment with MDMA. Phase shifts to a photic stimulus were significantly attenuated by pre-treatment with MDMA. Our study demonstrates that repeated exposure to MDMA may alter the ability of the circadian clock to phase shift to a photic and non-photic stimulus in the hamster. Disruption of circadian function has been linked with a variety of clinical conditions such as sleep disorders, mood, concentration difficulties and depression, consequently outlining the potential dangers of long-term ecstasy use.

Circadian phototransduction and the regulation of biological rhythms

The vertebrate circadian system that controls most biological rhythms is composed of multiple oscillators with varied hierarchies and complex levels of organization and interaction. The retina plays a key role in the regulation of daily rhythms and light is the main synchronizer of the circadian system. To date, the identity of photoreceptors/photopigments responsible for the entrainment of biological rhythms is still uncertain; however, it is known that phototransduction must occur in the eye because light entrainment is lost with eye removal. The retina is also rhythmic in physiological and metabolic activities as well as in gene expression. Retinal oscillators may act like clocks to induce changes in the visual system according to the phase of the day by predicting environmental changes. These oscillatory and photoreceptive capacities are likely to converge all together on selected retinal cells. The aim of this overview is to present the current knowledge of retinal physiology in relation to the circadian timing system.

Phenotypic rescue of a peripheral clock genetic defect via SCN hierarchical dominance

The mammalian circadian system contains both central and peripheral oscillators. To understand the communication pathways between them, we have studied the rhythmic behavior of mouse embryo fibroblasts (MEFs) surgically implanted in mice of different genotypes. MEFs from Per1(-/-) mice have a much shorter period in culture than do tissues in the intact animal. When implanted back into mice, however, the Per1(-/-) MEF take on the rhythmic characteristics of the host. A functioning clock is required for oscillations in the target tissues, as arrhythmic clock(c/c) MEFs remain arrhythmic in implants. These results demonstrate that SCN hierarchical dominance can compensate for severe intrinsic genetic defects in peripheral clocks, but cannot induce rhythmicity in clock-defective tissues.

Neurotensin phase-shifts the firing rate rhythm of neurons in the rat suprachiasmatic nuclei in vitro

The suprachiasmatic nuclei (SCN) of the hypothalamus house the main mammalian circadian pacemaker. Cell bodies in the rat SCN contain the neuropeptide neurotensin (NT), and two NT receptor types, NTS1 and nts2. Because the role of NT in the circadian rhythm processes is unknown, we studied the phase-shifting effects of NT on the firing rate rhythm of rat SCN neurons in vitro. Additionally, the NT receptor antagonists SR142948a and SR48692 were used to try and block any NT-induced phase shifts. To elucidate the second messenger pathway responsible for mediating the phase-resetting actions of NT, we utilized the phospholipase C (PLC) and protein kinase A (PKA) inhibitors U-73122 and KT5720, respectively. Application of NT during the projected day resulted in a large advance in the time of peak in FRR, whereas treatments during the projected night had no effect. Both NT receptor antagonists blocked the NT-induced phase shifts, as did the PLC inhibitor U-73122. The PKA inhibitor KT5720 had no influence on the magnitude of the phase shift caused by NT during the middle of the projected day. These results provide the first evidence that NT may play a role in regulating the rat circadian pacemaker, using NTS1 and nts2 receptors presumably coupled to PLC.

Three days of novel wheel access diminishes light-induced phase delays in vivo with no effect on per1 induction by light

The mammalian circadian clock, located in the hypothalamic suprachiasmatic nuclei, synchronizes endogenous behavioral and physiological rhythms to a 24 h period through responses to two types of stimuli: photic (light) and nonphotic (behaviorally induced arousal and/or increases in activity). Photic stimuli can block nonphotic effects and vice versa, although the mechanisms and levels of interactions between these two stimuli types are unknown. Here, we investigated whether 3 d of access to a novel running wheel alters the phase shift to light in vivo, and whether this effect could be seen on induction by light of the circadian gene per1. Through measurement of running wheel activity of golden hamsters, access to a new wheel for 3 d was shown to diminish photic phase delays with no effect on phase advances. As seen using in situ hybridization, however, there was no effect on levels of light-induced per1 mRNA. This study indicates a possible role for this paradigm as a model of interactions between photic and nonphotic stimuli.

Daily infusion of melatonin entrains circadian activity rhythms in the diurnal rodent Arvicanthis ansorgei

The effect of exogenous melatonin (MEL) on the circadian system in nocturnal species has been extensively studied, but little is known about its chronobiotic effect in diurnal mammals. The present study investigated the effect of exogenous MEL on the circadian locomotor activity rhythm in the diurnal rodent Arvicanthis ansorgei. Male animals (n=34) were fitted with a subcutaneous catheter for daily infusion of MEL (1 h; 100 microg) and their running wheel activity was recorded. The results showed that administration of MEL to animals free-running in DD entrained their activity rhythm by phase advances at circadian time (CT) 10.62, and by phase delays at CT -0.40 (CT 0, activity onset). The range of entrainment was 17 and 11.5 min for advance and delay stimuli, respectively. Interestingly, in the nocturnal rat and the A. ansorgei, entrainment of the activity rhythm to exogenous MEL by phase advances occurs at exactly the same phase of the circadian cycle. In both nocturnal and diurnal species, the sensitivity window for exogenous MEL is located near the activity/rest transition points. It is concluded that the functional properties of entrainment to exogenous MEL are similar to those of other nonphotic stimuli. Furthermore, A. ansorgei might be an interesting animal model for studies on the chronobiotic effects of exogenous MEL in diurnal mammals including humans.

Djungarian hamsters: a species with a labile circadian pacemaker? Arrhythmicity under a light-dark cycle induced by short light pulses

In most cases, phase-shifting effects of light pulses are studied in animals kept in constant darkness (DD) or in animals released into DD following the stimulus. In this study, the authors exposed Djungarian hamsters (Phodopus sungorus) to short light pulses during the dark phase of a 16:8 light-dark (LD) cycle and thus obtained a type VI phase response curve. Light pulses early in the night caused phase delays of the activity onset as well as phase advances of the activity offset, whereas light pulses later in the night resulted in phase advances of the activity offset only. A combination of two 15-min light pulses-the first one given late in the scotophase and the second given early in the dark phase of the following night-led to a strong compression of the activity phase alpha. In 75% of all animals, daily rhythms were no longer visible after complete alpha compression, and long-term arrhythmicity (up to 145 days) persisted despite continued exposure to an LD cycle. Because three independent output rhythms of the clock (i.e., activity, body temperature, and melatonin rhythms) were equally affected, the authors conclude that overt arrhythmicity was due not merely to disrupted output pathways but to an altered state of the central pacemaker. The authors suggest a qualitative two-oscillator model to explain this phenomenon. Their hypothesis assumes that, due to loose coupling, the pacemaker of Djungarian hamsters can be driven to a state of zero phase difference between the two oscillators, with zero amplitude of their outputs.

Phase shifting the hamster circadian clock by 15-minute dark pulses

The mammalian circadian pacemaker can be phase shifted by exposure to a period of darkness interrupting otherwise continuous light. Circadian phase shifting by dark pulses was interpreted originally as reflecting a photic mirror-image mechanism, but more recent observations suggest that dark pulse-induced phase shifting may be mediated by a nonphotic, behavioral state-dependent mechanism. The authors recently presented evidence indicating that the dark-pulse phase response curve (PRC) is in fact a complex function, reflecting both photic mirror image and nonphotic mechanisms at different phases of the circadian cycle. Previous studies of dark pulse-induced phase shifting have universally employed relatively long (2 to 6 h) pulses, which complicates PRC analysis due to the extended segment of the underlying PRC spanned by such a long pulse. The present study was therefore designed to examine the phase-shifting effects of brief 15-min dark pulses presented at both mid-subjective day and subjective dusk, and to explore the possible activity dependence of these effects by using physical restraint to prevent evoked locomotor activity. The results indicate that 15-min dark pulses are effective phase-shifting stimuli at both midday and dusk. Furthermore, as with longer dark pulses, phase shifting by 15-min dark pulses is completely blocked by physical restraint during subjective day but combines in a simple additive manner with the independent phase-shifting effect of restraint at subjective dusk.

Senescence, sleep, and circadian rhythms

The goal of this review article is to summarize our knowledge and understanding of the overlapping (interdisciplinary) areas of senescence, sleep, and circadian rhythms. Our overview comprehensively (and visually wherever possible), emphasizes the organizational, dynamic, and plastic nature of both sleep and circadian timing system (CTS) during senescent processes in animals and in humans. In this review, we focus on the studies that deal with sleep and circadian rhythms in aged animals and how these studies have closely correlated to and advanced our understanding of similar processes in ageing humans. Our comprehensive summary of various aspects of the existing research on animal and human ageing, both normal and pathological, presented in this review underscores the invaluable advantage of close collaboration between clinicians and basic research scientists and the future challenges inherent in this collaboration. First, our review addresses the common age-related changes that occur in sleep and temporal organization of both animals and humans. Second, we examine the specific modifications that often accompany sleep and CTS during aging. Third, we discuss the clinical epidemiology of sleep dysfunctions during ageing and their current clinical management, both pharmacological and non-pharmacological. Finally, we predict the possible future promises for complementary and alternative medicine (CAM) that pave the way to the emergence of a "Holistic Sleep Medicine" approach to the treatment of sleep disorders in the ageing population. Further studies will provide additional valuable insights into the understanding of both sleep and circadian rhythms during senescence.

Bimodal regulation of mPeriod promoters by CREB-dependent signaling and CLOCK/BMAL1 activity

Circadian rhythmicity in mammals is under the control of a molecular pacemaker constituted of clock gene products organized in transcriptional autoregulatory loops. Phase resetting of the clock in response to light involves dynamic changes in the expression of several clock genes. The molecular pathways used by light to influence pacemaker-driven oscillation of clock genes remain poorly understood. We explored the functional integration of both light- and clock-responsive transcriptional regulation at the promoter level of the Period (Per) genes. Three Per genes exist in the mouse. Whereas mPer1 and mPer2 are light-inducible in clock neurons of the hypothalamic suprachiasmatic nucleus, mPer3 is not. We have studied the promoter structure of the three mPer genes and compared their regulation. All three mPer promoters contain E-boxes and respond to the CLOCK/brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like protein 1 (BMAL1) heterodimer. On the other hand, only mPer1 and mPer2 promoters contain bona fide cAMP-responsive elements (CREs) that bind CRE-binding protein (CREB) from suprachiasmatic nucleus protein extracts. The mPer1 promoter is responsive to synergistic activation of the cAMP and mitogen-activated protein kinase pathways, a physiological response that requires integrity of the CRE. In contrast, activation of mPer promoters by CLOCK/BMAL1 occurs regardless of an intact CRE. Altogether, these results constitute strong evidence that CREB acts as a pivotal endpoint of signaling pathways for the regulation of mPer genes. Our results reveal that signaling-dependent activation of mPer genes is distinct from the CLOCK/BMAL1-driven transcription required within the clock feedback loop.

A neural clockwork for encoding circadian time

Many daily biological rhythms are governed by an innate timekeeping mechanism or clock. Endogenous, temperature-compensated circadian clocks have been localized to discrete sites within the nervous systems of a number of organisms. In mammals, the master circadian pacemaker is the bilaterally paired suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The SCN is composed of multiple single cell oscillators that must synchronize to each other and the environmental light schedule. Other tissues, including those outside the nervous system, have also been shown to express autonomous circadian periodicities. This review examines 1) how intracellular regulatory molecules function in the oscillatory mechanism and in its entrainment to environmental cycles; 2) how individual SCN cells interact to create an integrated tissue pacemaker with coherent metabolic, electrical, and secretory rhythms; and 3) how such clock outputs are converted into temporal programs for the whole organism.

Opposing actions of neuropeptide Y and light on the expression of circadian clock genes in the mouse suprachiasmatic nuclei

The circadian clockwork of the hypothalamic suprachiasmatic nuclei (SCN) is synchronized by light and by nonphotic cues. The core timing mechanism is cell-autonomous, based on an autoregulatory transcriptional/translational feedback loop of circadian genes and their products. This study investigated the effects of neuropeptide Y (NPY), a potent nonphotic resetting cue, and its interaction with light in regulating clock gene expression in the SCN in vivo. Injection of NPY adjacent to the SCN and transfer to darkness 7 h before scheduled lights out, shifted the circadian activity-rest cycle. Exposure to light for 1 h immediately after NPY infusion blocked this behavioural response. NPY-induced shifts were accompanied by suppression of both mPer1 and mPer2 mRNA in the SCN, assessed 3 h after infusion. mPer mRNAs were not altered 1 h after infusion. Levels of mClock mRNA or mCLOCK immunoreactivity in the SCN were not affected by NPY at either time point. In parallel to the behavioural response, the NPY-induced suppression of mPer genes in the SCN was attenuated when a light pulse was delivered immediately after the infusion. These results identify mPer1 and mPer2 as molecular targets for both photic and nonphotic (NPY-induced) resetting of the clockwork, and support a synthetic model of circadian entrainment based upon convergent up- and downregulation of mPer expression.

Neuropeptide Y rapidly reduces Period 1 and Period 2 mRNA levels in the hamster suprachiasmatic nucleus

The mammalian suprachiasmatic nucleus (SCN) contains the main circadian clock. Neuropeptide Y (NPY) that is released from the intergeniculate leaflet of the lateral geniculate body to the SCN, acts in the SCN to advance circadian phase in the subjective day via the NPY Y2 receptor. We used semi-quantitative in situ hybridization to determine the effect of NPY on circadian clock genes, Period 1 (Per1) and Period 2 (Per2), expression in SCN slices. Addition of NPY to the brain slices in the subjective day resulted in reduction of Per1 and Per2 mRNA levels 0.5 and 2 h after treatment. NPY Y1/Y5 and Y2 agonists decreased Per1 within 0.5 h. These results suggest that NPY may induce phase shifts by mechanisms involving or resulting in reduction of Per1 and Per2 mRNA levels.

The circadian clock and behavior

Temporal organization is a fundamental feature of all living systems. Timing is essential for development, growth and differentiation and in the mature organism, it is essential to maintain normal physiology and behavior. The biological entity that permits an organism's day/night organization is the circadian system. In the following, we describe how daily or circadian activity is measured in mice, and what such activity measurements can tell us about the state of the animal.

Cycle of period gene expression in a diurnal mammal (Spermophilus tridecemlineatus): implications for nonphotic phase shifting

Ground squirrels, Spermophilus tridecemlineatus, were kept in a 12:12 h light-dark cycle. As expected for a diurnal species, their locomotor activity occurred almost entirely in the daytime. Expression of mPer1 and mPer2 in the suprachiasmatic nucleus was studied at six time points by in situ hybridization. For both these genes, mRNA was highest in the first part of the subjective day (about zeitgeber time 5). This is close to the time when mPer1 and mPer2 expression is maximal in nocturnal rodents. These results have implications for understanding nonphotic phase response curves in diurnal species and thereby for guiding research on nonphotic phase shifting in people.

Distribution of substance P and neurokinin-1 receptor immunoreactivity in the suprachiasmatic nuclei and intergeniculate leaflet of hamster, mouse, and rat

The circadian pacemaker in the hypothalamic suprachiasmatic nuclei (SCN) receives photic information directly via the retinohypothalamic tract (RHT) and indirectly from retinally innervated cells in the thalamic intergeniculate leaflet (IGL) that project to the SCN. Using standard immunohistochemical methods, we examined the presence and distribution of substance P (SP) and the neurokinin-1 receptor (NK-1) in the SCN and IGL of rat and determined whether the patterns of immunostaining generalized to the SCN and IGL of Syrian hamster, Siberian hamster, and mouse. Terminals immunoreactive for SP were sparse within the SCN of Siberian and Syrian hamsters and mouse but were intense in the ventral, retinally innervated portion of the rat SCN. Immunostaining for the NK-1 receptor was mainly absent from the SCN of hamster and mouse. In contrast, a plexus of NK-1-ir cells and processes that was in close proximity to SP-ir terminals was found in the ventral SCN of the rat. Substance P-ir terminals were observed in the IGL of all four species, as were NK-1-ir cells and fibres. Double-labelled IGL sections of hamster or rat revealed SP-ir terminals in close apposition to NK-1-immunostained cells and/or fibres. These data indicate that SP could be a neurotransmitter of the RHT in rat, but not in hamster or in mouse, and they highlight potential species differences in the role of SP within the SCN circadian pacemaker. Such species differences do not appear to exist at the level of the IGL, where SP-ir and NK-1-ir were similar in all species studied.

Opposing effects of behavioural activity and light on neurons of the suprachiasmatic nucleus

The mammalian circadian pacemaker is located in the suprachiasmatic nuclei. It can be shifted in phase by photic cues and by the behavioural activity of the animal. When presented together, light and behavioural activity attenuate each others' phase-shifting effect. Still unclear is how behavioural activity affects the suprachiasmatic nuclei and how it interacts with photic information. Previously, we reported the occurrence of behaviourally induced suppressions of neuronal activity. The present study investigates the characteristics of these suppressions as a function of circadian time and, additionally, in the presence of photic cues. We performed long-term multiunit activity recordings of neurons in freely moving rats and found that these suppressions of neuronal firing in the suprachiasmatic nucleus occurred at every phase of the circadian cycle. The magnitude of the suppressions showed a circadian variation, with larger suppressions during subjective day. When a light pulse was applied during a suppression, light and activity appeared to oppose each others' effects within the recorded population of neurons. The resulting discharge level appeared to be the sum of both responses. The opposing effects of light and activity were also found in single unit recordings, indicating that photic and behavioural stimuli interact at the level of a single neuron.

Patterns of wheel running are related to Fos expression in neuropeptide-Y-containing neurons in the intergeniculate leaflet of Arvicanthis niloticus

A variety of nonphotic influences on circadian rhythms have been documented in mammals. In hamsters, one such influence, running in a novel wheel, is mediated in part by the pathway extending from neuropeptide-Y (NPY)-containing cells within the intergeniculate leaflet (IGL) of the thalamus to the hypothalamic suprachiasmatic nucleus (SCN). Arvicanthis niloticus is a species in which all individuals are diurnal with respect to general activity and body temperature when they are housed without a running wheel, but access to a running wheel induces a subset of individuals to become nocturnal. In the first study, the authors evaluated the possibility that nocturnal and diurnal patterns of wheel running in Arvicanthis are correlated with differences in IGL function. Adult male Arvicanthis housed in a 12:12 light-dark (LD) cycle were monitored in wheels, classified as nocturnal or diurnal, and then perfused either 4 h after lights-on or 4 h after lights-off. Sections through the intergeniculate leaflet were processed for immunohistochemical labeling of Fos and NPY. The percentage of NPY cells that expressed Fos was significantly influenced by an interaction between time of day and phenotype such that it rose from night to day in diurnal animals, and from day to night in nocturnal animals. In the second experiment, the authors established that running in a wheel actually induces Fos in the IGL of Arvicanthis. Specifically, the proportion of NPY cells expressing Fos was increased by access to wheels in nocturnal animals at night and in diurnal animals during the day. In the third experiment, the authors established that lesions of the IGL eliminate NPY fibers within the SCN, suggesting that these IGL cells project to the SCN in this species as has been established in other rodents. Together, these data demonstrate a clear difference in NPY cell function in nocturnal and diurnal Arvicanthis that appears to be caused, at least in part, by the differences in their wheel-running patterns, and that NPY cells within the IGL project to the SCN in Arvicanthis.

Independence of circadian timing from cell division in cyanobacteria

In the cyanobacterium Synechococcus elongatus, cell division is regulated by a circadian clock. Deletion of the circadian clock gene, kaiC, abolishes rhythms of gene expression and cell division timing. Overexpression of the ftsZ gene halted cell division but not growth, causing cells to grow as filaments without dividing. The nondividing filamentous cells still exhibited robust circadian rhythms of gene expression. This result indicates that the circadian timing system is independent of rhythmic cell division and, together with other results, suggests that the cyanobacterial circadian system is stable and well sustained under a wide range of intracellular conditions.